Four-wire transistorized conference call telephone system



Dec. 31, 1963 J. E. COX

FOUR-WIRE TRANSISTORIZED CONFERENCE CALL TELEPHONE SYSTEM Filed Feb. 6, 1961 De. 31, 1963 J. E. cox 3,116,369

FOUR-WIRE TRANSISTORIZED CONFERENCE CALL TELEPHONE SYSTEM Filed Feb. 6, 15,361 I5 Sheets-Sheet 2 ilO/ dl//ffiX/'! 295 11. :Mw ai; l- 292 Dec. 31, 1963 J. E. Cox 3,116,369

FOUR-WIRE TRANSISTORIZED CONFERENCE CALL TELEPHONE SYSTEM Filed Feb. 6, 1961 3 Sheets-Sheet 3 302 I572 gui/l2 United States Patent 3,516,369 UUR-vVlRlE TRANSBTREZED CONFERENCE CALL TELEPHNE SYSTEM John E. Cox, Passaic, NJ., assigner to international Telephone and Teiegraph Corporation, a corporation of Filed Feb. 6, 196i, Ser. No. 37,415 9 Claims. (Si. 179-l) This invention relates to four-wire telephone systems and more particularly to conference call circuits for use in such systems.

Conference call circuits commonly use either hybrid arrangements for effectively isolating the transmitters of a subscriber station from the receivers of the same station or common bus bars for transmitting signals and common bus bars for receiving signals, in lwhich case isolation between the .ttnansmit and the receive circuits is provided by amplifiers coupled between the common bus bars. These methods of isolation are used in an effort to overcome some of the difficulties inherent in conference call circuits. Among these difficulties are singing, echo, and attenuation caused by feedback between the transmitter and receiver of the common station, by improper impedance terminations between stations, by improper impedance matching, `and by power drain due to the number of telephones connected to a common circuit.

When hybrid circuits are used to minimize these ditliculties, new problems are introduced because it is necessary to balance the hybrid network against lthe subscriber lines. When common buses are utilized for minimizing these difliculties, it has been necessary to use complicated and expensive amplifiers in conjunction with the buses.

Accordingly, it is an object of this invention to provide new `and impnoved fourwire conference call circuits.

Ano-ther object of this invention is to provide tnansistorized conference call circuits that do not utilize either hybrid networks or common buses joined by complicated and expensive amplifiers.

Yet another object is to terminate each transmitter and receiver used in such circuits by its charateristic impedance.

A further object of this invention is to provide an inexpensive and economical conference call circuit. In this connection, an object of this invention is to provide conference call circuits wherein singing is eliminated, echo is minimized, and loss due to attenuation is compensated for by inexpensive ampliliers.

In -accordance with one aspect of this invention, a conference call circuit is provided with n number conference channels, each comprising a transmit circuit and a receive circuit. Common to the output of the transmit circuit of each channel are fz-l amplifiers each of which features a negative feedback arrangement. The feedback stabilizes the amplifiers and allows use of unmatched, inexpensive commercial grade components. The transmitter of each conference channel is individually connected to all the receivers of the other channels through the lz-l ampliers, there being one amplier per connected receiver. 'Ilhe number of amplifiers is n-l because there is no connection in the conference circuit between the transmit circuit and the receive circuit of the same conference channel.

The above mentioned and other objects of this invention, together with the manner of obtaining them 'will become more apparent, and the invention itself will be best understood by making reference to the lfollowing description of a preferred embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FG. la shows by block diagram a conference call circuit utilizing the invention in connection with automatic switching equipment;

ICC

FIG. 1b shows by block diagram ta conference call circuit utilizing the invention in connection with manual equipment;

FIG. 2 is a schematic diagram showing a preferred embodiment of the invention used in the conference channels shown in FIG. la;

FIG. 3a is ta schematic circuit diagram sho-wing a preferred embodiment of the invention used in the conference channels shown in FIG. 1b; and

FIG. 3b is a schematic circuit diagram of the power supply utilized by the circuits of FIGS. 2 and 3a.

Where possible, simple terms are used and specific items are described hereinafter to facilitate an understanding of the invention; however, it should be understood that the use of such terms Iand reference to such items are not to act in any manner as a disclaimer of the full range of equivalents which is normally given by established principles of patent law. 'Io illustrate, the attached drawings show stepping switches which are used when the conference call circuit is automatically connected, whereas any type of automatic switches such as crossbar switches may be utilized. Quite obviously, other examples could be selected to illustrate the manner in which speciiic terms that have been used and items which have been described are entitled to a wide range of equivalents.

BRlEF DESCRWTON An examplary telephone sys-tem incorporating the principles of this invention used in connection with automatic switching equipment is included in FlG. lez. There is sho-wn a telephone conference call circuit comprising a plurality of automatic switch trains, subscriber stations, and an attendants cabinet providing means `for the attendant to set up the conference call by interconnecting the subscribers through conference call channels and the automatic switch trains. Each of the conference channels has associated therewith la lamp in the attendants cabinet to aid in the supervision by the attendant of the conference connection. The conference channels include a tnansmit portion and a receive portion. The transmit portions of each of the channels contains a multiplicity of 'amplifiers such as amplifiers through which lthe transmit portion of the channel is interconnected to the receive portions of all the other channels in the conference call circuit. The number of amplifiers per channel is :one less than the number of channels since the transmit portion and the receive portion of the same channels are not interconnected. The ampliiiers thereby function to isolate the transmit and receive portion of the channels as well as to provide the necessary power to permit transmission of speech with negligible loss.

In operation, a calling subscriber at a four-wire station A is connected to a four-wire station B via la Well known step-by-step type switch train including lineiinder, selector, second selector, and connector. :lt should be understood that each of the arrows 5 8 :which interconnect the switches in FIG. la indicates a number of brush and bank terminals that yare sufficient to extend the fourwire telephone circuit and control connections. Thus, subscriber A initiates a call by removing a handset to operate the linender and thereafter dials the directory number of called station B to set the seleoton second selector, land conneotor. The connector tests the called line tand rings station B if it is then idle.

In carrying out this invention, the calling subscriber may also extend a connection through `a portion of the automatic switch train to `an attendant cabinet 9` if la conference call is desired. More specifically, the calling subscriber A removes a receiver or handset and dials a predetermined digit, such as 0, to drive the selector to a preselected level giving access to the attendant cabinet 9 via a six wire line itil including a transmit pair T, receive pair R, and a control pair of private or slee-ve conductors Pi, P2. The attendant at cabinet 9 talks to the subscriber at station A and learns that a conference call between A and stations C F is desired. Thereafter, the attendant connects an idle conference channel to a second selector by way of patch cord Zti interconnecting jack pairs such `as E8, l@ which are individual to the conference channels and second selectors. Then the attendant having access to one of the desired subscribers dials out over the connected second selector such as shown in switch trains M-lfi, on a one at a time basis, to seize and signal each of the subscriber stations C F in a conventional manner.

To conference -the stations A and the called stations C F the attendant at cabinet t) with patch lcords such as cord 20 completes talking channels between each of the called stations and a common link i in the conference call equipment 17. Each `of the trunk lines associated with the attendant trunk level 0^ terminates in a pair of jacks similar to jacks It, 19.

There ybeing a transmit jack T and a receive jack R in each case. Thus, the attendant uses a patch cord such `as cord Ztl to interconnectrthe subscriber station C and Van idle conference call channel such as No. 1 on an individual basis by interconnecting,7 jacks i8, 19. In a similar manner, other patch cords (not shown) interconnect the noted subscriber stations A and D-F through conference call channels No. 2-No. 5. Quite obviously key controlled circuits could be substituted for the patch cords.

Each of ythe conference call channels includes a transmit circuit land a receive circuit, such as 22, 23. Each transmit circuit includes a number ot amplifiers Ql-Qid which increase signal strength to compensate vfor the added lines. The inputs of each transmit circuit and the outputs of each receive circuit are connected to the subscriber lines as by jacks 18, 19 and patch cord Zit. The outputs of the transmit channels and the inputs lof the receive channels are interconnected by the common channel 15, here shown as a number of conductors or busses. Thus, each subscriber line is connected to all other lines in the conference call circuit by Iw-ay of these conductors.

Means are provided for isolating the Itransmit circuit of the conference channels from the receive circuit of the same channel. Referring specifically to transmit circuit 2i, by way of example, the isolation means are shown generally as four amplifiers Q11, Ql, Q23 and QM through which the transmit circuit is connected to the receive circuit of the other four conference channels No. 2-No. 5. Each `amplifier includes negative feed-back to provide stabilization and also to make it possible to use inexpensive transistors and other components. It is the unidirectional characteristics of these amplifiers that almost `completely isolates the transmit from receive portions of each channel. Because of the isolation, the circuit eliminates singing and minimizes echo.

`Power transfer is maximized because each circuit is terminated by a characteristic impedance, with provisions for automatically inserting and removing such impedance as necessary. Any attenuation caused by the inherent adverse conditions of the conference call circuit is compensated for by `the amplification provided by the amplifiers.

It should be noted that the isolation referred to above is a result of the circuit-ry wherein each ampli-tier is connected to only one receiver. Also, because the amplifiers are substantially unidirectional, there is no path whereby the transmitted signal can be received in the receive circuit of the conference call channel that contains the active transmitter. 1

The isolation means, such as amplifiers Qdi-Q14, tnerefore tend to eliminate singing and minimize echo. At

the same time, the amplification ofthe isolation means obviates the attenuation caused `by such things as the multiplicity or" phones included in the conference circuit and provides ya conference connection with substantially zero loss.

FIG. 1b shows another embodiment of the invention in block diagram form wherein the circuits are manually conferenced. More specically there is shown a conference call circuit that is manually set up by the operator at :opera-tor position 5t) by such means as jacks or keys. The circuit indicates a plurality of subscribers A-F individually connected to the operators position. Also individually connected to the operators position are an equal number of conference channels Nos. 1-5. The conference channels are divided into transmit portions, such as T51, and receive portions, such as R52, of conference channel No. 1. Each of the transmit portions is connected to the receive portions of .all the other conference channels but not to the receive portion of its own channel. Thus, transmit portion T51 is not connected to receive portion RSZ. The operator manually connects the subscribers individu-ally to a conference channel at the request of any of the subscribers. The details of the conference channels are the same as the conference channels of FIG. la. The supervisory circuits, however, iare diterent as will become apparent from the detailed description to follow.

DETAILED DESCRIPTTON For a more complete understanding of the invention, reference is made to the schematic circuit diagrams of FIGS. 2 and 3 which show in detail the component parts of the conference channels of FIGS'. la 4and lb respectively.

T/ze automatically Connected conference call Girault-A plurality of conference channels No. 1, No. 2 and No. n 'are shown in FIG. 2. In each there is ia transmit or send channel 22 and a receive channel Z3. Connecting means such as jacks Mn, 1%, are provided `tor joining the transmit and the receive channels to the attcndants positions.

In greater detail, FIG. 2 shows the transmit jack 19a as connected to tip and ring leads TS1 and R51 in the transmit channel ZZ. Also, the receive jack 19k is connected to the tip and ring leads TRl and RRE in the receive circuit 23. The tip and ring leads in the conference call channel No. 2 `are connected to the attendants position at jacks we, 19d; and the tip and ring leads in the channel No. n are connected at jacks 19m, 1911.

Chain circuit means are provided for connecting the attendant cabinet with each of the subscriber lines connected to jacks 19a, 1%, 19C, 19d and 19m, 1911. More specifically, at the attendant cabinet are "n number single pole, double throw, talking key switches SWU), SW(2) SWUt). Terminal 1 of switch SWU) is connected to terminal 2 of switch .Svi/(2); terminal l is connected -to terminal 2 of the next switch until terminal l of switch SWOFI) is connected to terminal 2 of switch SW(n). Terminal 1 of switch SW(n) is not connected to anything. Terminal 2 of switch SWU.) is grounded. To switch the attendants telephone set into communication with each of the subscriber stations, the switch SW in a particular channel is thrown to position "3 where a relay, such as relays Zitti, 231%` 26d", is operated. Since terminal 2 of each of the switches is connected to the armatures of those switches and, further, since terminel 2 of switch SWU) is the onlyground source associated with `any fof the switches, only one relay of the aforementioned group of relays can be operated at a time; therefore, the attendant lcan only communicate with one subscriber at a time. For example, with relay 200 opera-ted, communication is through `contacts Zila, 262.51, 203 and 205. At this same time, corresponding contacts 4in channels No. 2No. n are open because the chain circuit to relays '230, 26B* is broken at contacts l, 2 of switch SWG).

Responsive to the operation of switch SWU.) armature to terminal 3i, relay 206 operates over a circuit that can be traced from ground through terminal 2, the armature, and terminal 3 of switch SWG), and the coil of relay 2th) to battery.

Responsive to the operatic-n of relay 213e, makebefore break contacts 25.1151. close to complete a talking circuit from the tip lead (not shown) in the subscribers line circuit through the automatic switch train, the attendants cabinet via a patch cord, jacks 19a, lead TS1, contacts 201m, lead TS(c), jack 1911 to the attendants headset 21. Make-beforebreak contacts 202a complete the talking circuit from the ring lead (not shown) in the subcribers line circuit through the automatic switch train, the attendants cabinet 9 via a patch cord, plugs and jacks 19a, lead R81, contacts 211211, lead RS(c) and jack 19R to the attendants headset 21. Break contacts 20111, 292]] open the talking circuit through conference channel No. l. Contacts 2&3, 205 close similar talking circuits from the subscriber station to the attendants headset 21. Contacts 2114 open to `remove the ringback tone by opening the tone circuit from the ringback tone source RBT.

The operation of switches Sil/(2) through SWW) has the same effect on their respective conference channels as switch SW( 1) has on channel No. l. Each of the aforementioned switches is normally maintained in position whereby the armature is in contact with its terminal 1 unless the attendant desires to communicate with the subscriber attached to a particular conference channel and thus the attendant is limited to communicating `with only one subscriber at a time.

Means, such as relay 211i, are provided for indicating that the conference channel is connected to an automatic switch train. Relay 210 is operated by a loop battery and ground coming from the automatic switch train via the tip and ring of jack 19a. in a manner which is well known in the art of automatic telephony.

Responsive to the operation of relay 211i, contacts 211 close to operate relay 29d over a circuit that can be traced from ground in receive channel 23 through the filament of lamp LP1, line L(1), contacts 223, 211 and the upper or high resistance winding H of relay 2% to battery. The current iiowing through the circuit just traced is insuiiicient to light lamp LP1 due to the high resistance in the circuit of the coil of relay 290.

When relay 29u operates, contacts 291 close to complete a hold circuit for relay 291i` that can be traced from ground through the iilamcnt of lamp LP1, line 1,(1), contacts 223, 211 and 291 and the lower or low resistance winding L of relay 2591i to battery. The current ilowing in this circuit is sufficient to cause lamp LPl to light and thereby indicate that the conference channel No. l is connected to an automatic switch train. Also responsive to the operation of relay 29?, contacts 292 close to extend a resistance battery marking to a night alarm NA lead over an obvious circuit. Conference channels No. 2 through No. 11 contain indicating means similar to relay 211B comprising relays such as 241i and 27 0 and associated circuitry.

Means are provided for blocking the direct current utilized to operate the indicating means. The direct current is blocked to prevent shorting the coil of relay 210 through the primary winding of transformer T11. That is, as here shown, the capacitors C12, C13 block direct current in the transmit side of the conference channel No. l, while similar capacitors are utilized in conference channels No. Z-No. 11.

This embodiment of the invention shows the transmit and receive means shunted by terminating impedance such as R11, R12, R21, R22, R111 and R112. These impedances are equivalent to the circuit characteristic impcdance of the receiver and the transmitter and function to provide maximum power transfer, minimize echo and also, in the case of R12, R22, R112, permit idle channels to soak up the signal power that is normally transmitted to the subscriber.

Answer supervision is provided when relay 220 in conference channel No. 1 operates by the battery reversal ti which occurs when an associated line is olf-hook. Iri greater detail, relay 220 is operated over a circuit that extends from battery through a winding on relay 220, the tip conductor TR1 and from ground through the other winding on relay 22? to the ring conductor RR1.

Responsive to the operation of relay 220, contacts 221, 222 open to remove the terminating impedance resistors R11, R12 that shunt the transmit and receive circuits of conference channel No. l when that channel is not in use. Also responsive to the operation of relay 22d, contacts 223 open to extinguish lamp LP1, thus indicating to the attendant that the called party has answered. Channel No. 2 discloses relay 251i and channel No. n discloses relay 230 to accomplish in their associated channels the functions of relay 220.

Means are provided for extending ringback tone to the calling subscriber during the period of waiting for all subscribers to answer. The tone circuit in conference channel No. l can be traced from a ringback tone source RBT through coupling capacitor C11, contacts 2114, 29661 to tip lead TR1 of the receive circuit to the calling subscriber. The ringback tone circuit of the other conference channels are identical.

Coupling means are provided between the portion of the transmit circuit of the conference channel that is connected to the jacks on the attendants cabinet and the portion of the transmit circuit that is connected to the receive circuits in the other conference channels. In conference channel No. l the coupling means is transformer T11; in channel No. 2, transformer T21; and in channel No. n, transformer T111. In a similar manner the receive circuits in the conference channels contain coupling means such as transformer T12 conference channel No. l, transformer T22 in conference channel No. 2 and transformer T112 in conlference channel No. n. The transformer provide impedance matching, isolation and amplification.

The secondary windings of each transformer' are permanently shunted with matching impedance such as resistors R13` and R18 in conference channel No. l, resistors R23, R28 in conference channel No. 2 and resistors R113, R118 in conference channel No. n. These resistors also maximize power transfer and reduce reliections.

In accordance with one aspect of this invention, a number (rz-1) isolating means are provided in the transmit circuits of each of the n conference channels. For example, transistor ampliers Q11, Q12, Q13 and Q14 are in conference channel No. l; transistor amplifiers Qnl, Qn2, Qn3 and Q114 are in conference channel No. n. In an exemplary circuit these transistors were type RCA 2N408. The transmit circuits, such as 22, of each of the conference channels is connected to the receive circuits, such as 23, of each of the other conference channels through one of the transistor amplifiers. The unidirectional characteristics of the transistors assist in isolating the transmit circuit from the receive circuit and thereby reduce singing and echo to a minimum since there is no feedback path from the receiver to the transmitter. At the same time the amplification provided by the transistors tends to cancel any inherent attenuation in the circuit. Moreover, these amplifiers provide the necessary sparation between transmit and receive signals without requiring hybrid networks which are dimcult to balance, especially when used with a number of telephone lines having impedances that vary from connection to connection as is common with switched circuits.

Means are provided for stabilizing the amplifiers and thus making the amplifier circuit amenable to the use of low cost components and providing inexpensive and economical conference call circuits. This is accomplished by utilizing degenerative feedback supplied by coupling a relatively low value resistance in the emitter circuits of each of the transistors. The feedback resistors are R14, R15, R16 and R17 in conference channel No. l; R241, R25, R26 and R27 in conference channel No. 2; and

aliases a R(n)4, R(n)5, R(n)6 and R(n)7 in conference channel No. n.

Means are provided whereby the conference call circuit is completed responsive to the attendants operation of a set-up switch SVt/(c). In greater detail, when switch SW(c) is operated to the conference position, relay 295 is operated over an obvious circuit.

Responsive to the operation of relay 295, contacts 296a open to disconnect the ringback tone source from conference channel No. 1. Contacts 296i; close to complete the receive circuit from transformer T12 to the tip and ring loop of a subscriber line. In a similar manner, contacts 297er, 29'7b, 299e and 29% operate to disconnect ringback tone and complete the receive circuits in their particular conference channels.

Also responsive to the operation of relay 295, contacts 293 close to complete the circuit that supplies power to the transistor ampliiiers. In greater detail, a voltage divider circuit is completed when contacts 298 close. The circuit can be traced from battery through resistor RC1, contacts 29S, resistors RC2, and RC3 to ground. A tap -C between contacts 298 and resistor RC2 provides collector voltage and is connected to the midpoints of one Winding on each of the transformers T12, T22, T112 in the receive circuits of the conference channels. A tap labeled -B between resistors RC2, RC3 is connected to similar points on transformers T11, T21, T111 in the transmit circuits of the conference channels. Filter capacitors CC1, CC2 shunt the C and -B voltage taps to provide a low impedance A.C. path to ground to keep voice currents out of the D.C. supply.

With the foregoing description of the component parts of the circuit in mind, it is thought that the invention may be best understood by the following description of the manner in which the circuit operates.

Setting up the conference (automatic acc`ess).-A subscriber requiring a conference call dials the connect code and is connected by the switch train to the attendants board. Responsive to this connection a lamp (not shown) on the attendants board lights up to notify the attendant, who then obtains information as to the directory nurnbers of the parties to be called. Ii the call is to be set up immediately, the attendant cross patches the calling subscriber to conference channel No. 1 through jacks 19a and 1911 by means of suitable cords. A ground forwarded over thesleeve lead of jack 19a operates a relay (not shown) in the automatic switch train. Responsive to the operation of that relay, relay 211i operates from batery and ground potentials returned over the ring and tip leads RR1 and TR1. A loop condition is also eX- tended over leads TR1 and RR1 to operate relay 220 as long as the subscriber remains off-hook.

Responsive to the operation of relay 21), contacts 211 close preparing a circuit for operation of lamp LP1 and relay 290 over a circuit that has previously been traced. These components do not operate, however, as long as there is an off-hook condition which operates relay 224D.

Responsive to the operation of relay 2211, contacts 221, 222 open to remove resistors R11, R12 from the circuits and contacts 223 open and thereby prevent the immediate operation of lamp LP1.

The attendant then proceeds to connect the remainder of the parties. Ringback tone is returned to the subscriber via capacitor C11, contacts 204 and 296a to inform him that the conference is being set up.

Assuming that conference channel No. 2 is used to connect with a called subscriber, then that channel is cross connected to a suitableoutgoing automatic line by means of patching cords inserted in jacks 19C and 19d. Switch SW(2) is operated causing relay 230 to operate because SW(1) of the chain is unoperated. Responsive to the operation of relay 230, the attendants headset and dial circuit are connected to the automatic outgoing line via jacks 19C and 19d and contacts 231, 23202), 233, and 235. The attendantthen dials the required number. The dial pulses are extended over tip and ring leads TR(c),

8 RR(c), TR2, RRZ, jack 19d to van automatic switch train such as switch trains 11, 14 (FIG. la). The called station is rung in a conventional manner.

When the called party answers, a loop is completed over tip and ring leads TR2 and RRZ through the automatic switch train to operate relay 25), over a circuit that extends from ground on the upper coil of relay 25@ through ring lead RRZ, jack 19d, the automatic switch train and battery is supplied through the lower coil of relay 251i and conductor TR2. Responsive to the operation of relay 2511, the characteristic impedance resistor R21 is removed from the transmit portion of the circuit No. 2 by the opening of contacts 251. The characteristic impedance R22 in the receive circuit is removed when contacts 252 open. The circuit for supervisory lamp LPZ is opened by the operation of contacts 253. When this lamp is exextinguished the attendant knows that the called subscriber station is connected. He then requests the subscriber to wait until the other conference parties are called, and restores the talk key SW(2) to its normal position. This releases relay 23@ which causes relay 24) to operate and the attendants dial and headset to be disconnected both over circuits previously traced.

The attendant then proceeds to call the other conference parties in an identical manner, until all have answered.

When all the parties have been called, the attendant operates the means for completing the conference call circuit shown here as the conference setup switch SW(c) which operates relay 295 over an obvious circuit.

Responsive to the operation of relay 295, the ringbaclc tone source RBT is disconnected at contacts 29M, 29161, and 299m Each receive channel is connected to the associated transformer at contacts 295i?, 29712, and 299i. The conference call circuit is now completed and ready for use.

At any time during the call the attendant can intercept and converse with any channel by operating the appropriate switch. For example, the attendant can intercept and converse with channel No. 1 by operating switch SW(1) thereby extending ground through the coil of relay 2W. Relay Ztl@ removes ringback tone source RBT and disconnects the tip lead TS and ring lead RS from relay 210. The attendants headset is connected directly to the subscriber over a circuit that can be traced from the attendant headset 21 through jacks WR, 19S over tip and ring leads TS(c), RS(c), TR(c) and RR(c), contacts 20111, 211251, 203 and 2114, respectively, tip and ring leads TS1, R81, TR1 and RR1 respectively, jacks on the attendants board 19a and 19t), and the automatic switch train to the calling subscriber station. The leads TS(c), RS(c), TR(c) and RR(c) are the common sending and receiving leads that appear on the attendants cabinet in this exemplary four-wire conference call circuit.

Operation of the conference call circuit-When the rst subscriber is talking, his speech currents are transmitted his line circuit, the automatic switch train and tip Iand ring leads TS1 and R81 to isolating and amplifymg means (here shown `as transistor ampliers Q11, Q12, Q13 and Q14). The input or base electrode of each amplifier connects to a `bus lthat is connected across one side of transformer T11 and in series with a characteristic impedance terminating means such as R23. The output or collector electrode of each amplifier is connected to yan individually associated bus in group 15. Therefore, variations caused by voice modulation of the base current of 4transistors Q11, Q12, Q13 and Q14 cause a larger Variation in the collector current of these transistors and in the current on the buses 15. However, the voltage drop across the emitter resistors R14, R15, R16 `and R17 tends to increase as the emitter-collector current through the transistors increases, thereby causing the emitters to become more negative and, consequently, lowering the potential difference between the emitter and the base which in turn tends to decrease the current. Therefore, the amplification is Alessened'but the stability 'aliases Q of the amplifiers is increased. yIn other Words, the resistors generate negative feedback in that they tend to reduce an increasing current or to increase a decreasing current.

The collector C of transistor Q11 is connected to terminal 3 of transformer T22, lthe recive transformer of conference channel No. 2. Thus, the voice circuit can be traced from ground through resistor R14, the emitter E and collector C of transistor Q11, bus a, terminal 3, the upper right-hand winding and terminal 4 of transformer T22 to the C terminal of the common power supply, contacts 298 and resistor RC1 to battery. The current in the upper right-hand winding of transformer T22 induces a current in the left-hand winding of T22, which current flows through a circuit comprising contacts 297 b, tip and ring leads TR2 and RRZ, jack 19d, a patch cord similar to cord 2t) (FIG. la), and through an automatic switch train 11 to subscriber B. Capacitors C24 and C are blocking capacitors to keep direct current out of the transformers and to prevent the transformer winding from short circuiting the relay 256.

In a similar manner, the collectors of all the transistors in each of the transmit channels is connected to a terminal 3 on the secondary winding of a receive transformer in another conference channel.

Thus, isolating and amplifying means, such as the individual transistors Q11-Q14, are used to connect each of the transmit portions of the conference channels to the receive portions of every other conference channel but not to the receive portion of the conference channel in which the transistors are located. Consequently, there is no feedback path in the conference channels between the transmitter and the receive portions of the same conference channels. Generally, therefore, there are n number conference channels, and n l transistors, all of which serve to eliminate singing by isolating the receiver from its associated transmitter and to compensate for attenuation by amplifying the signals emitted from the transmit portions of the conference channel.

Clearing the conference-When the rst conference party clears, relay 220 releases but relay 210 remains operated from the ground and battery that yare respectively on the top and ring leads TS1 and R81 of the transmit portion of the conference channel. The release of the relay 220 reconnects the terminating impedances R11, R12 across the channel transformers T11, T12 permitting the remainder of the conference to proceed if required. The release or the relay 221] also causes the supervisory lamp LP1 to be connected in series (but not lit) with the alarm relay 290 via contacts 221, 223, which relay is thereby operated. Responsive to lthe operation of relay 290, contacts 2%1 close to complete a circuit which lights lamp LPI via the low resistance coil of relay 290 to battery. This informs the attendant to release this connection by removing the patch cords.

ln a similar manner each of -the other participants in the conference call replaces his handset and lthe supervisory light in the attendants cabinet associated with the conference channel to which they were connected lights up over the circuit analogous to that previously traced. When the supervisory lamps are lit, the attendant removes the patching cords and resets the setup switch Svi/(c). The conference circuit is disconnected and the switch train released.

Setting up the conference (manual access) -Reference will now be made to FIG. 3a which shows the circuit details utilized when conference calls are set up manually. When an operator receives a request for a conference call setup, the required parties are called from an operator position 5t) and then the conference circuit is set up by connecting the appropriate lines to the conference circuit via means such as patching cords, for example.

Each conference call channel includes a transmit side and a receive side as shown at 51, 52. The transmit side S1 includes as principal components a coupling transformer such as T318, a number (n l) of transistor amplifiers, and relay 31N). The receive side 52 also includes a coupling transformer such as T31R which is connected vto the output electrode of a transiter amplifier in each of the other conference call channels. Thus, the receive transformer T31R connects to the collector of the transistor amplifier Q321 and of the transistor amplifier Q3n1. The resistors R313, R315 provide impedance matching, resistors R311, R312 provide negative feedback and resistor R314 provides la termination when the channel is idle 4to soak up the signal power normally delivered to a subscriber connected to the channel.

Each conference channel appears on a separate twow-ay jack such as 3331, 1R31 of a toll board and the operator by inserting the yappropriate cord circuit plug into the two-way jack can connect a party into the conference through a conference channel.

When a subscriber line is plugged into a conference channel through jacks such as 183i and 1R31, relay 33d is operated from battery in the subscribers line circuit that is applied to the sleeve of fthe jack. Responsive to the operation of relay 311), contracts 391 open to remove the terminating impedance resistor R314 from the circuit. Also responsive to the operation of relay 3in1, contacts 3112 (FlG. 3b) are operated to a closed position causing battery to be applied to common power supply terminals B and C. `Similar relays in channels No. 2 and No. n control contacts 312 and 3n2. Terminal C is connected between the contacts of the relays and resistor R2 while terminal B is connected between resistors R2 and R3 which form a voltage divider.

Battery is applied to the base electrodes of the transistors in channels No. l-n over a circuit that can be traced from terminal B through the right-hand windings of transformers T315, T328 and T3nS to a common bus to which the bases or" the transistors are attached. The collectors are supplied from terminal C through the right-hand windings of the transformers T31R, T32R and TSnR. The emitter of each transistor is connected to ground -through a negative feedback resistor, such as R311, which has a stabilizing effect as described previously in this specification.

Operation of the manually connected conference circuit-Speech currents liowing through the send conductor such as T531, RS31, TS32, R332, TS3n and RS311 are amplied by the single stage transistor amplifiers, such as Q311, Q312 in channel No. l; Q321, (2322 in channel No. 2, and Q3n1 and (23112 in channel No. n. Through these transistor amplifiers the signals are transmitted to the other channels in the conference circuit. Means for stabilizing the amplifiers are provided, such as resistors R311, R312 in channel No. 2; and R3n1 and RSnZ in channel No. n. These resistors in the emitter circuit of these transistor amplifiers provide negative feedback, thereby reducing the amplification but producing a much more stable output, that is not adversely affected by such things as temperature variations. The transmit sections of the conference channels are connected to the receive sections of all the other conference channels via common conductors 15'. There is no connection in the conference channel between the transmit and receive portions of the channel. Any sidetones the talking subscriber receives are provided by circuitry in his individual telephone set. This circuitry could be any of the circuits commonly in use for such purposes.

Clearing tile manually connected conference call circuit.-When the parties in the conference call return their handsets to the associated hookswitches, onahcok supervision is received and cord circuit supervisory lamps light (not shown but similar to the supervisory lamps LP depicted in FIG. 2). The operator then takes down the patch cords that completed the conference connection. This causes the relays in the conference channels to release and the circuit is cleared.

While the principles of the invention have been shown l i and described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

i claim:

1. in a telephone system for extending connections between calling and called ones of a plurality of subscriber stations, means for establishing a telephone conference call circuit comprising iz number subscriber stations, lz number conference call channels, each of said channels having transmit means and receive means, 11-1 number of isolating means associated with each of said transmit means, said transmit means in each of said channels being connected to each of said receive means in other of said channels through individual ones of said isolating means, each of said isolating means comprising a negative feedback stabilized transistor amplifier, and means including attendant operated means for individually connecting each of said n number subscriber stations to one of ysaid conference channels to set up said conference call circuit.

2. In a telephone conference call circuit of clairn l wherein the means for connecting each of said n number subscriber stations to one of said conference call channels comprises automatic switching means.

3. in a telephone conference call circuit of claim 1 wherein the means for connecting each of said zz number subscriber stations to said conference channels comprises manual switching means.

4. A telephone conference call circuit comprising n conference call channels, each of said channels having transmit means and receive means, and 1z-1 isolating means associated with each of said transmit means, said transmit means in each of said channels being connected to each of said receive means in other of said channels through one of said isolating means whereby the receive means is effectively isolated from the transmit means of the same channel and singing is prevented.

5. A telephone conference call circuit comprising n subscriber stations, an operators position, n conference call channels, each of said channels havinT transmit means and receive means, means for connecting each of said channels to an operators position, means for connecting each of said n subscriber stations to said operators position, means within said operators position for connecting each of said n subscriber stations to each of said n conference channels, 11-1 isolating means associated with each of said transmit means, said transmit means in each of said channels being connected to each of said receive means in other of said channels through one of said isolating means thereby conferencing all of said n subscriber stations through each of said n conference channels.

6. A telephone conference call circuit comprising n channels, each of said channels having transmit means and receive means, n-l isolating means associated with each of said transmit means, means for connecting said transmit means in each of said channels to each of said receive means in other of said channels through one of said isolating means whereby the receive means is eitectively isolated from the Vtransmit means of the same channel and singing is prevented.

7. In a telephone system for extending connections between calling and called ones of a plurality of subscriber stations, means for establishing a telephone conference call circuit comprising n number of said subscriber stations, n conference call channels for interconnecting said n subscriber stations, each channel comprising receive means for receiving communication signals originating at said stations and transmit means for transmitting communication signals originating at said stations, a plurality of isolating means associated with each transmit means, means for connecting each of said transmit means of said channels to ail ofthe other channels through said plurality of isoiating means, means for connecting cach of said n subscriber stations to a different one of said n channels to establish said conference call circuit, a plurality of terminating impedance means, and means for connecting said impedance means to ones ot said channels responsive to ones of said n subscriber stations disconnecting from said onesof said channels thereby maintaining a constant characteristic impedance for said conference call circuit.

8. In a telephone switching system for extending connections between calling ones and called ones of subscriber stations, said switching system comprising an attendants cabinet, means comprising n number conference call channels for establishing a conference call circuit between n number of said subscriber stations, means for connecting said channels to said attendants cabinet, common link means for interconnecting said channels, chain circuit means in said attendants cabinet for individually connecting an associated one of said 11 number subscriber stations to an idle one of said n number channels thereby conerencing said n number of subscriber stations, said chain circuit comprising n number single pole doubie throw switches each having an armature and a pair of poles with said armature normally connected to a first one of said poles, means for individually connecting the first pole of each switch to the armature of a preceding one of said switches, means for connecting the armature of a first one of said switches to ground thereby grounding the armature on each normal one of said switches and means responsive to operating any of said switches to connect the grounded armature to the other pole for coupling the attendants cabinet to the subscriber station associated with said operated switch via a portion of said individually connected conference call channel.

9. A teiephone conference cali circuit comprising n number of channels, each of said channels having transmit means and receive means for transmitting and receiving conference communication signals, 11-1 transistor amplitiers in each of said transmit means, means for interconnecting said channeis including means for coupling an output electrode from each of said ampliiiers to an individually associated one of said receive means in another channel, and means for stabilizing said transistor arnpliiiers comprising a series resistor coupled into the emitter circuit causing degenerative feedback which increases the stability of the transistor amplifier and thereby allows the utilization of low priced components.

References Cited in the le of this patent UNITED STATES PATENTS 

7. IN A TELEPHONE SYSTEM FOR EXTENDING CONNECTIONS BETWEEN CALLING AND CALLED ONES OF A PLURALITY OF SUBSCRIBER STATIONS, MEANS FOR ESTABLISHING A TELEPHONE CONFERENCE CALL CIRCUIT COMPRISING N NUMBER OF SAID SUBSCRIBER STATIONS, N CONFERENCE CALL CHANNELS FOR INTERCONNECTING SAID N SUBSCRIBER STATIONS, EACH CHANNEL COMPRISING RECEIVE MEANS FOR RECEIVING COMMUNICATION SIGNALS ORIGINATING AT SAID STATIONS AND TRANSMIT MEANS FOR TRANSMITTING COMMUNICATION SIGNALS ORIGINATING AT SAID STATIONS, A PLURALITY OF ISOLATING MEANS ASSOCIATED WITH EACH TRANSMIT MEANS, MEANS FOR CONNECTING EACH OF SAID TRANSMIT MEANS OF SAID CHANNELS TO ALL OF THE OTHER CHANNELS THROUGH SAID PLURALITY OF ISOLATING MEANS, MEANS FOR CONNECTING EACH OF SAID N SUBSCRIBER STATIONS TO A DIFFERENT ONE OF SAID N CHANNELS TO ESTABLISH SAID CONFERENCE CALL CIRCUIT, A PLURALITY OF TERMINATING IMPEDANCE MEANS, AND MEANS FOR CONNECTING SAID IMPEDANCE MEANS TO ONES OF SAID CHANNELS RESPONSIVE TO ONES OF SAID N SUBSCRIBER STATIONS DISCONNECTING FROM SAID ONES OF SAID CHANNELS THEREBY MAINTAINING A CONSTANT CHARACTERISTIC IMPEDANCE FOR SAID CONFERENCE CALL CIRCUIT. 